In a mobile wireless communication system, for example, in a call between a train which is a moving object, and the ground, switching between wireless zones (base station zones) each of which is managed by a wireless base station occurs during the call because the train moves at a high speed. In order to reduce sound interruptions which occur at the time of this switching, for example, patent reference 1 discloses a technique of, at the time of a call start, blocking, in advance, both a channel of a base station zone located in the traveling direction with respect to a base station zone to which the train currently belongs, and a channel of a base station zone located in a direction opposite to the traveling direction, and similarly passing a voice signal which is transmitted to the voice channel used in the current base station zone also to the next base station zone.
There are two reasons why a channel of the base station zone located in the traveling direction of the train is blocked in advance. One of them is that the voice signal is also passed to the next base station zone, as mentioned above. The other one is that because there is a limit to the number of channels usable in one base station zone, there may occur a problem that, when the train has moved to the next base station zone, there is no empty channel for the call, and therefore the channel is blocked to avoid this problem. Further, the reason why a channel of the base station zone located in a direction opposite to the traveling direction is blocked is that when there is a train which is moving to this base station zone from the opposite direction and is during a call, the number of channels required for calls needs to be prevented from becoming insufficient when the train has reached this base station zone.
On the other hand, a VoIP (Voice over IP) technique of IP-packetizing and transmitting a voice signal has become popular with the development of IP (Internet Protocol) networks in recent years. Introduction of VoIP is expected also in mobile wireless communications from this time forward. In VoIP, voice information is transmitted by using IP packets without being discriminated from data information. Therefore, there is provided an advantage of being able to implement integration of data communications and voice communications at a low cost.